With the advent of fiber-bodied composite cans adopted as a low-cost container for certain products such as flavored drink concentrates and other products where the container is normally opened with a conventional can opener, it has been a constant problem for the fiber can industry to develop a can which would permit easy opening with the various types of can openers presently in use. The paperboard material usually used in fiber-bodied composite cans is softer, more compressible and thicker than the sheet metal material used in metal bodied cans. These different properties combine with the metal end conventionally used on fiber bodied cans to produce a crimp or seam which is somewhat wider and more compressible than the seam of metal bodied cans. Conventional can openers were originally designed for use with metal bodied cans which inherently have a narrower stronger bead and also a hard metal wall to produce a positive supporting surface which prevents penetration into the body wall by the toothed driving wheel in properly oriented driving relationship with the underside of the can bead, thus preventing undercutting and slippage of the driving wheel with respect to the bead. With the softer fiber body of composite cans, the driving wheel engages both the bead and adjacent outer body wall, and on certain can openers tends to penetrate into the body wall causing the driving wheel to undercut the bottom edge of the bead.
In the United States a great number of inexpensive can openers are sold each year. Many of these can openers are not sufficiently well made to work properly, even when new. Typically, they do not stand up under the heavy usage given by the modern day housewife in this country. Even if these can openers don't work properly on metal bodied cans, the housewife continues to use them; and when used on fiber bodied cans which provide less support than the metal bodied cans, slippage problems occur sufficiently often to be objectionable. Composite can opening problems have also occurred to a minor degree with certain types of more expensive can openers due to the specific construction and arrangement of the driving and cutting parts of such openers so that the opening of a composite can with the can openers presently in use has been a problem for fiber can openers.
Obviously, the problems encountered in opening the metal end closures of composite can construction are quite different from the problems encountered in opening cans having metal side wall construction. However, in both instances, when employing the conventional can opener devices to open the cans, it will be apparent that the driving force produced by the driving wheel against the can body and the underside of the bead must always be greater than the resistance of the can opener to travel around the can during the cutting operation. Thus, if this resistance can be reduced, less driving force will be required, and the possibility of the toothed driving wheel slipping on the body and underside of the bead and, thus, losing its drive, will be substantially reduced.
A number of factors influence the resistance to travel of the can opener around the top of the can. One of these factors is the resistance to cutting or breaking through of the metal itself. Another, is the frictional resistance exerted by surfaces of the bead on the elements of the can opener respectively engaged therewith. This materially increases the drag and resistance to movement of the can opener around the can which, in turn, causes slippage of the driving wheel, since the total resistance to travel becomes greater than the driving force produced by the driving wheel of the can opener. If the total resistance to travel (the resistance to cutting or breaking through the metal plus the drag resistance exerted by the can opener elements riding on the engaged surface of the bead) is less than the driving force produced by the driving wheel, the can will be easily opened by the can opener.
The concept of employing a weakening line on an end structure for a container to facilitate the opening thereof, is not basically new. The U.S. patents to J. A. Geiger, Nos. 3,362,569, 3,362,570; J. Henchert No. 3,073,480; Ellerbrock No. 3,397,809; and No. 3,929,251, Urmstrum, relate to container structures having a weakening or score line formed in the end closures to facilitate the opening thereof either with the use of conventional can opener, or by some other built-in easy-opening feature.
However, as the use of the composite can becomes more and more of an accepted manner of packaging, the manufacturing costs become more and more of a consideration. Furthermore, as production requirements increase, so does the utilization of energy. Accordingly, in addition to the mechanical and physical features of the can construction, it is an extremely important aspect of any such container, or container component, to be as inexpensive as possible so as to justify mass production.
In a number of prior art endeavors to form weakening lines in the metal end closures, extrusion or coining steps have been utilized. It will be appreciated that such a displacement of metal, particularly where dealing with steel sheet, by extrusion, requires extensive amounts of pressure by the forming presses. It has been found that a shearing or blanking operation to form the weakening line requires considerably less pressure and, thereby, reduces the cost of manufacture of the resultant end closure.